如何在Kotlin中创建一个不可变列表，而在Java中它也是一个不可变列表? [英] How to create an immutable list in Kotlin that is also an immutable list in Java?

问题描述

我有一个Java/Kotlin互操作问题. Kotlin不可变列表被编译为可变的普通java.util.ArrayList！

I have a Java/Kotlin interop problem. A Kotlin immutable list is compiled into a normal java.util.ArrayList that is mutable!

科林(图书馆):

class A {
  val items: List<Item> = ArrayList()
}

Java(消费者):

Java (consumer):

A a = new A();
a.getItems().add(new Item());   // Compiles and runs but I wish to fail or throw

从Java角度来看，如何也使我的Kotlin类完全不可变?

How to make my Kotlin class fully immutable from Java perspective too?

推荐答案

默认情况下，Kotlin中的所有非Mutable____集合都是编译时只读类型，但不是不可变的.请参见以下代码段:

All non-Mutable____ collections in Kotlin are compile time read-only types by default, but not immutable. See the following code snippet:

fun main(args: Array<String>) {
  // Explanation for ArrayList(listOf()) later down the post
  val list: List<Int> = ArrayList(listOf(1, 2, 3))
  println(list) // [1, 2, 3]

  // Fails at compile time
  // list.add(4)

  // Uh oh! This works at runtime!
  (list as MutableList<Int>).add(4)
  println(list) // [1, 2, 3, 4]
}

要真正拥有一个不变的列表，请考虑番石榴的Immutable____集合:

To truly have an immutable list, consider Guava's Immutable____ collections:

import com.google.common.collect.ImmutableList

fun main(args: Array<String>) {
  val list: List<Int> = ImmutableList.of(1, 2, 3)
  println(list) // [1, 2, 3]

  // Fails at compile time
  // list.add(4)

  // Fails at runtime, as expected
  (list as MutableList<Int>).add(4)
  println(list) // [1, 2, 3, 4]
}

请注意，Kotlin的某些标准运行时函数可能会返回不可修改，不可调整大小的集合，因此，当您直接将只读集合转换为可变集合时，所有押注都会被取消.

Be aware that some of Kotlin's standard runtime function may return collections that are either unmodifiable, not resizable, etc., so all bets are off when you directly cast a read-only collection to a mutable one.

例如，当前listOf()(将来可能会更改！)会通过Arrays.asList(T...)在vararg参数数组周围返回java.util.Arrays.ArrayList.可以修改此列表"，但是永远不能添加或删除元素，因为您不能调整数组的大小.参见 Arrays.asList(T...) javadoc 了解更多信息.

For example, listOf() currently (this may change in the future!) returns a java.util.Arrays.ArrayList around the array of vararg parameters via Arrays.asList(T...). This "list" can be modified, but elements can never be added or removed, as you cannot resize an array. See Arrays.asList(T...) javadoc for more information.

如果您确实想要任何给定集合中的可变集合，请考虑使用.toMutableList()进行复制.这将适用于任何集合:

If you really want a mutable collection from any given collection, consider making a copy using .toMutableList(). This will work on any collection:

import com.google.common.collect.ImmutableList

fun main(args: Array<String>) {
  val list: List<Int> = ImmutableList.of(1, 2, 3)

  val copy = list.toMutableList()
  copy.add(4)

  println(copy) // [1, 2, 3, 4]
  println(list) // [1, 2, 3]
}

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